Seems like a decent PSU and should last you for a while without having to worry. Right well the only way we are going to get to the bottom of this is top go right back to basics.I think the best thing is to remove the motherboard from the case and then connect it up on the desktop (on some cardboard or similar). All you want to start with is the PSU, motherboard, CPU/heatsink/fan and the buzzer. When you have done that try and POST. Obviously it won't complete but it should give some beeps.
It is late now so I am going to bed but will be back in the morning to continue.